Rapid Prototyping Methods

Why This Matters

Rapid prototyping sits at the heart of modern product development—it's how you transform CAD models into physical objects you can hold, test, and iterate on. You're being tested on more than just knowing what each method does; you need to understand when to choose one method over another, what trade-offs each involves, and how material properties and production volume influence your decision. These concepts appear repeatedly in design justification problems and process selection questions.

The methods covered here span additive manufacturing (building up material layer by layer), subtractive manufacturing (removing material from a solid block), and formative processes (shaping material through molds or heat). Don't just memorize the names—know what principle each method demonstrates, what constraints it solves, and where it fails. That's what separates a strong exam response from a mediocre one.

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Additive Manufacturing Methods

These processes build parts by adding material layer by layer, enabling complex internal geometries that would be impossible with traditional methods. The key trade-off is between surface finish, material strength, and build speed.

3D Printing (Fused Deposition Modeling)

• Layer-by-layer extrusion of thermoplastic filament—heated material is deposited through a nozzle, fusing to the previous layer as it cools
• Material options include PLA, ABS, and PETG, each offering different properties: PLA for ease of use, ABS for heat resistance, PETG for chemical resistance
• Lowest barrier to entry for rapid prototyping, making it ideal for early-stage concept validation and iterative design cycles

Stereolithography (SLA)

• UV laser cures liquid photopolymer resin into solid plastic, producing parts with exceptional surface finish and fine detail
• Resolution typically 25-100 microns, far exceeding FDM capabilities—critical for prototypes requiring tight tolerances or aesthetic evaluation
• Higher material costs and post-processing requirements (washing, curing) make it better suited for small, detailed parts than large structural components

Selective Laser Sintering (SLS)

• Laser fuses powdered polymer (typically nylon) without requiring support structures—unfused powder supports the part during printing
• Produces mechanically robust parts suitable for functional testing, load-bearing applications, and end-use components
• Ideal for complex assemblies and interlocking parts since no supports means no removal marks or weak attachment points

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Subtractive Manufacturing Methods

Subtractive processes remove material from a solid workpiece to achieve the final geometry. These methods excel at precision and material variety but generate waste and struggle with internal features.

CNC Machining

• Computer-controlled cutting tools remove material from metal, plastic, or composite stock with tolerances as tight as $\pm 0.001"$
• Works with engineering-grade materials including aluminum, steel, titanium, and high-performance plastics—critical when prototype material must match production material
• No layer lines or anisotropic properties, producing parts with consistent mechanical behavior in all directions

Laser Cutting

• High-powered laser vaporizes material along a programmed path, creating precise 2D profiles in sheet stock
• Kerf width (material removed by the laser) typically ranges from $0.1-0.3 \text{ mm}$, requiring compensation in design files
• Excellent for flat components, enclosures, and brackets—combine with bending or welding for 3D assemblies

Sheet Metal Fabrication

• Combines cutting, bending, and joining operations to transform flat metal sheets into structural components
• Bend radius and k-factor calculations determine how flat patterns unfold—understanding these is essential for accurate designs
• Produces lightweight, high-strength parts commonly used in enclosures, brackets, and chassis assemblies

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Formative and Molding Processes

These methods shape material using molds, heat, or pressure. They bridge prototyping and production, offering economies of scale but requiring upfront tooling investment.

Injection Molding

• Molten plastic injected under high pressure into a steel or aluminum mold cavity, then cooled and ejected
• Per-part cost decreases dramatically with volume—tooling may cost $\$5,000-\$100,000+$, but parts can cost pennies each at scale
• Design for manufacturability (DFM) is critical: uniform wall thickness, draft angles, and gate placement determine success

Vacuum Forming

• Heated thermoplastic sheet draped over a mold and drawn tight using vacuum pressure—simple tooling enables fast turnaround
• Limited to single-sided geometry with relatively uniform wall thickness; undercuts require multi-part molds
• Cost-effective for large enclosures and housings where surface detail is less critical than size and speed

Urethane Casting

• Liquid polyurethane poured into silicone molds created from a master pattern—bridges the gap between one-off prototypes and injection molding
• Silicone molds typically yield 20-50 parts before degradation, making it ideal for pilot runs and market testing
• Wide range of durometers and properties available, from rigid plastics to flexible rubber-like materials

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Hybrid and Accelerated Approaches

Rapid Tooling

• Creates production-capable molds using additive or soft-tooling methods, dramatically reducing lead time from weeks to days
• Bridges prototyping and production by enabling short runs (50-500 parts) without full hard-tooling investment
• Common approaches include 3D-printed mold inserts, machined aluminum molds, and silicone tooling—each with different life expectancy and tolerance capabilities

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Quick Reference Table

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Self-Check Questions

1. Which two additive methods both use lasers but produce parts with fundamentally different mechanical properties? Explain why.

2. A client needs 75 plastic housings that closely mimic injection-molded ABS for a trade show in three weeks. Which method would you recommend, and what are the key trade-offs?

3. Compare CNC machining and FDM for creating a functional prototype that must withstand repeated loading. What factors would push you toward each method?

4. Why does injection molding have high initial costs but low per-part costs? How does this cost structure influence when you'd choose it over urethane casting?

5. You're designing a sheet metal bracket that will be laser cut and bent. What design considerations (name at least two) must you account for that wouldn't apply to a 3D-printed version of the same part?